Web-like materials, such as paper, paperboard, and the like typically are required to meet particular mechanical property specifications. Normal quality control techniques require testing of the web-like materials to ensure that the web uniformly meets the desired mechanical property specifications.
Destructive-type tests are known for measuring mechanical properties of such web-like materials. Such destructive-type tests are normally conducted off-line on representative samples of the web. There are various problems with such off-line destructive-type testing. For example, such testing is relatively time-consuming and requires production to be sampled periodically when product is received from the machine. In addition, due to the destructiveness of such testing, it is normally performed on representative samples of the web, which may be taken, for example, every several thousand square feet of material. In such a situation, a substantial amount of waste is incurred if the web-like material is found to fail the test.
To solve the problems associated with such destructive-type test measurements of mechanical properties of web-like materials, ultrasonic testing techniques have been developed. Such testing is performed on-line and thus is relatively quicker than off-line destructive-type testing. In addition, ultrasonic testing provides relatively continuous indication of various mechanical properties of the web-like material to assure virtually uniform quality of the product while minimizing waste.
In known ultrasonic systems for testing various mechanical properties, two ultrasonic transducers are provided. The ultrasonic transducers are generally disposed on opposing sides of the web to allow ultrasonic signals to be transmitted in a direction generally normal to the plane of the web. In such a system, one transducer acts as a transmitter while the other transducer as a receiver. The time of flight of an ultrasonic signal through the thickness of the web and the thickness of the web itself are obtained to determine various mechanical properties of the web. In particular, the time of flight of the ultrasonic signal through the thickness of the web is determined by measuring the time of flight of the ultrasonic signal between the transducers during a condition when there is no web or sample present and then measuring the time of flight of the ultrasonic signal during a condition when a web is disposed between the transducers. Consequently, the difference in the times of flight during the two conditions is representative of the time of flight of the ultrasonic signal through the web.
A method and apparatus disclosed in U.S. Pat. No. 5,493,911 ("'911") only provide what is effectively an average ultrasonic measurement over one or more tire circumference lengths. The '911 system requires an electronic rotational synchronization circuit to ensure that each measurement actually represents an average of measurements through one or more rotations of the pair of rotatable tires. The averaging and the rotational synchronization are necessary because this method is unable to account for variations in tire thickness along the circumference of the tire which lead directly to errors in specimen thickness and ultrasonic transit time measurements. This averaging process effectively averages the tire thickness variation out of the measurements. Consequently, the measurements are then distributed over sizable lengths along the specimen, thereby foregoing any chance of maintaining localized measurements of small specimen areas of the sheet.
Also, the nature of the use of the off-sheet calibration in the '911 system is limited due to the assumption that the two tires remain in the same relative rotational synchronization during on-sheet specimen measurements and off-sheet calibration. This assumption implies that the tires would need to have the same circumference, which is a condition that generally is difficult to achieve. As the two tires rotate slowly out of relative synchronization, the quality of the off-sheet calibration tends to degrade.
Prior systems also suffer from the problem that while wheel rotation variance can be partially alleviated by averaging, no instantaneous determination can be made. This limitation creates uncertainty in determining whether the wheel rotation variation has been taken into account when determining a transit time or caliper measurement for a sheet. This uncertainty is due in part to the inability to directly determine the initial and ongoing characteristics associated with the two wheel system--for example, the speed of propagation of ultrasonic pulses through the liquid.
In these systems, a solid delay line is used to roughly measure the speed of propagation through the liquid. The time of transit of ultrasound through the delay line is affected to some extent due to temperature changes of the liquid. These systems, while better than not having any compensation, are not completely error free. For instance, the temperature of the solid delay line naturally tends to lag the temperature of the liquid when the temperature of the liquid is changing. Without knowing the speed of propagation through the liquid to a high degree of certainty, it is difficult to infer the exact length of the fluid path within the tires from pulse reflection times within the solid delay line, and in turn, make accurate measurements of the transit time through the specimen or the specimen thickness (caliper).
What is needed then is a system for accurately and rapidly determining the transit times through all of the media through which the ultrasonic pulses are being propagated so that the transit time through the specimen can be accurately determined. Also, what is needed is a system for accurately and rapidly determining the path lengths through all of the media through which the ultrasonic pulses are being propagated so that accurate determinations of specimen thickness can be made. Finally, what is needed is a system for determining the transit time of ultrasound through specimens that is independent of the velocity of ultrasound through the fluid.